I have just unfortunately been through similar after making a new UC claim last month after receiving a migration notice.
The response or lack of from messages on the journal is poor but when i've contacted them over the phone, there's not much of a wait time but usually you'll get through to someone miserable and snotty.
I gave all the correct information when making our new claim and by the time the first statement came in, they still hadn't done the child verification part which meant there was no child element on our first payment, no disabled child element and because of the children missing, they subsequently reduced the HB too as they said we had unoccupied bedrooms.
It took several phone calls and a complaint to get it sorted out. It's frustrating that they are telling people that they must move over to UC but at the same time, they can't complete new claims in a reasonable timeframe.